From 2010-2014 we conducted a prospective observational study during 4 consecutive RSV seasons at Nationwide Children’s Hospital, Columbus, Ohio, USA. Previously healthy children <2 years of age with a first episode of RSV infection were enrolled either at the outpatient clinics (‘outpatients’) or within 24h [17-39h] (median [IQR]) of admission in the pediatric ward or the pediatric intensive care unit (PICU) (‘inpatients’). Asymptomatichealthy controls were enrolled during routine primary care visits or elective surgery not involving the respiratory tract. For study criteria see Supplementary methods E1. In addition to the need for hospitalization, RSV disease severity was assessed using a clinical disease severity score (CDSS), and by the need for supplemental oxygen, PICU admission and length of stay.(16)​